Activated carbons are well known and widely used for the removal of organic contaminants, chlorinated hydrocarbons, and free chlorine from drinking waters. Activated carbon powders are generally employed in static, batch processes. In a dynamic flow system, however, granulated activated carbons are required in order to provide useful flow rates. A column packed with granulated activated carbon can be used in a flow system which has the advantage of high levels of contaminant removal since the contaminant loading passes through the column as a wave.
When contaminated water is passed through a bed of granulated activated carbon, a wave or mass transfer zone is formed in the bed by continuous adsorption of contaminants. The contaminants are rapidly adsorbed in the front layers of the bed until the amount is in equilibrium with the inlet contamination concentration. At this point, the front layer is loaded to capacity and this zone of the bed is exhausted. Below the exhausted zone additional dynamic adsorption occurs in a second zone This zone is the mass transfer zone and its depth is controlled by factors such as nature of the contaminants, characteristics of the adsorbent, and hydraulic factors related to fluid flow. Once formed, the mass transfer zone moves down the bed until it reaches the outlet, whereupon the outlet concentration of contaminants rises sharply.
To effectively utilize a bed of granulated activated carbon adsorbent, it is essential to have a narrow mass transfer zone so that a sharp wave will pass through the bed and not allow early leakage of contaminants. To achieve this with granulated activated carbon, it is critical to have favorable adsorption dynamics, particularly fast transport of contaminants into the pores of the granule together with high adsorption capacity of the carbon powder within the granule.
In accordance with this invention, a granulated activated carbon is prepared by the steps of (1) agglomerating the activated carbon powder in an aqueous slurry to produce spherical granules having a particle diameter of from about 0.17 mm to about 0.71 mm with a mean value of about 0.43 mm; (2) separating and drying the granules; and (3) activating the granules by treatment with steam at a temperature of from about 105.degree. to about 120.degree..
To provide the desired adsorptive and flow properties, the major portion of the granules consists essentially of an activated carbon powder having a surface area of from about 2800 to about 3500 m.sup.2 /gm, an iodine number of from about 2500 to about 3800 mg/gm, a total pore volume of from about 1.0 to about 2.8 cc/gm, and a bulk density of from about 0.27 to about 0.32 gm/cc. A particularly useful active carbon can be prepared from coal coke, petroleum coke, or a mixture thereof by heating with hydrous potassium hydroxide at a first lower temperature and then at a second higher temperature to yield a very high surface area active carbon powder having a cage-like structure exhibiting microporosity. The preparation of such carbons is disclosed in Wennerberg, U.S. Pat. No. 4,082,694 which patent is incorporated herein by reference. The carbon powder has extremely high adsorptive capacity for water contaminants, and can be successfully employed in static batch processes to remove contaminants.
Because of its fine particle size, however, the above-described carbon does not provide satisfactory flow rates in a dynamic flow system. Agglomerating the carbon powder to produce granules can be effected by conventional methods utilizing clay, lignin, and such binders as disclosed, for example, in Lloyd, U.S. Pat. No. 3,352,788. Granules so prepared generally provide satisfactory flow rates in dynamic water treating systems; the adsorptive capacity of the carbon, however, is seriously deteriorated. This is particularly true for very high surface area carbons where the binder will frequently plug the small micro-pores where much of the surface area lies.
A critical feature of this invention is an agglomeration process which produces spherical granules that provide excellent flow rates and, at the same time, an essentially undiminished adsorptive capacity. The agglomeration process comprises mixing with good agitation an aqueous slurry containing from about 10% to about 15% by weight of carbon powder with from about 40% to about 60% by weight, based on the weight of the carbon, of a hydrocarbon bridging liquid together with up to about 10% by weight, based on the weight of the carbon, of a binder to improve granule strength. The bridging liquid is preferrably a hydrocarbon such as, for example, hexane, heptane, naphtha, kerosene, or toluene. The binder is suitably a clay or mineral such as, for example, dolomite, kaolin, talc, and the like. Polymeric binders, in amounts ranging from about 5 to about 25%, based on the weight of the carbon powder, can also be employed. Examples are the known water-insoluble adhesive polymers and copolymers of acrylic esters, methacrylic esters, acrylamides, and methacrylamides. Lignins modified, for example, with hexamethylene tetramine serve as suitable binders. Water-soluble, thermosetting polymers can also be used, for example, melamine-formaldehyde compositions as disclosed in Lloyd, U.S. Pat. No. 3,544,502. When the desired particle size distribution of from about 0.17 mm to about 0.71 mm is reached, the granules are separated and dried to remove most of the bridging liquid and water. The general process for obtaining spherical granules is disclosed in Farnand et al., U.S. Pat. No. 4,029,567 which patent is incorporated herein by reference.
The dried granules are then activated by treatment with steam, suitably under a pressure of about 5 psig, corresponding to a temperature of about 109.degree. C., for a period of about 45 minutes. In the agglomeration and activation process, the pore size distribution of the carbon particles within the granule remains essentially unaltered. In particular, the small micro-pores in the virgin powder, having a pore diameter of &lt;10 A.degree. and responsible for most of the surface area, are retained, as well as the meso-pores (20-200 A.degree. diameter) and macro-pores (&gt;1000 A.degree.) which provide effective transport of contaminants into the particle. Thus the agglomerated granules exhibit a trimodal pore volume distribution, i.e. peak volumes in each of the micro-pore, meso-pore, and macro-pore ranges. The presence of significant pore volumes in the meso- and macro-pore ranges provides an open pore structure which permits ready access of contamination to the adsorbing micro-pore structure and results in rapid and efficient attainment of adsorption equilibrium.